You are quite correct. When I got my PhD in chemistry, I was required to teach undergraduate chemistry labs. The native born students typically could not form a coherent sentence and their writing was the worst chicken scratch I've ever seen. The foreign students were far more literate, neat and organized in their writing. In freshman chemistry, there were about 120 students. In sophomore chemistry, the number dropped to about 25. The only foreign students that dropped were the ones that had minimal English skills. Most of the student that dropped were native ones. They simply could not handle the workload, stress, the requirement to calculate the correct answer and the fact you had to know the previous chapter in order to do the next. Some American students actually complained that the second mid-term was cumulative. They though it wasn't fair to be tested on material they had already been tested on. They didn't understand they needed to know that material in order to do the more advanced work!
On the other hand, the foreign students were paying 5 times the in state tuition rate and knew there'd be h*ll to pay from their parents if they didn't do well. Couple that with the strong work ethic found in most Asian countries and you have foreign students that do rather well compared to American students.
I had that type of experience as a foreign student as well. Imagine taking a test where the review was done in class and consisted of the teacher READING questions straight from the test and giving you the answer! People still did poorly simply because they were too lazy to listen or too busy partying to show up for the review. I found that the pervasive attitude was one of having a good time. As a foreign student I didn't have that luxury, my tuition, books, room and board cost my parents dearly. You are right, there would have been big trouble if I did not perform to the best of my ability.